The invention relates to a method of manufacturing a dispenser cathode comprising a porous cathode body which predominantly consists of a refractory metal or refractory metal alloy and being provided with a top layer which differs from the rest of the cathode body, the method comprising pressing a powder predominantly comprising a refractory metal or refractory alloy to form a body.
Dispenser cathodes of this type are used in electron guns for electron tubes such as television picture tubes, picture pick-up tubes, travelling-wave tubes, clystrons, transmitter tubes and the like.
Dispenser cathodes comprise a reservoir of emitter material to provide an adequately low exit potential for electrons at emitting surface. Dispenser cathodes of the type described in the opening paragraph comprise a porous impregnated body, having a top layer which differs from the rest of the body. It is desirable for the top layer to have properties which are advantageous for the emission of electrons, while the rest of the body has properties which are advantageous for the storage of emitter material. To that end, in said dispenser cathodes, the top layer is formed by a layer which has a composition and/or porosity differing from the rest of the body. The porosity of the top layer and of the rest of the body determine the total maximum quantity of emitter material to be stored in the body, the active surface area and the diffusion rate of active elements from the cathode body to the emissive surface. A low porosity of the top layer combined with a high porosity of the rest of the body combines a relatively slow diffusion rate of active elements to the emissive surface with a relatively large storage capacity, which has a favourable effect on the operating life of the cathode. It is alternatively possible to provide a top layer with emission-stimulating material (e.g. Sc.sub.2 O.sub.3). In U.S. Pat. No. 4,625,142, a cathode body is pressed from a tungsten powder, on which, prior to pressing, a 0.2 mm thick layer of a mixture of 95% by weight of tungsten powder and 5% by weight of scandium oxide powder (Sc.sub.2 O.sub.3) is provided. After pressing and sintering, the cathode body consists of an approximately 0.1 mm thick scandium oxide-containing, porous tungsten layer having a density of approximately 83% of theoretical density, on a 0.7 mm thick porous tungsten layer having a density of approximately 75% of theoretical density. Compared with cathodes of a homogeneous composition, that is to say cathodes which are provided with emission-stimulating material throughout the entire cathode body, this has the advantage that a greater quantity of emitter material can be stored.
It is a disadvantage of the prior art method that it is of poor utilization for the manufacture of cathodes whose emitting surface is not flat, but curved. On compressing powder provided with a top layer of a different composition in a press the die of which has a curved surface, it was found experimentally that, after pressing, the top layer was not uniformly distributed over the surface, but was shifted to the side edges of the mould. This results in a non-uniform by distributed emission of electrons, immediately or after some time in the future.